Skid steering basics
- Based on fixed direction wheels[1] or tracks
- There are two groups or sides of wheels: left and right
- All wheels in a group MUST always point in the same direction
- All wheels in a group MUST always rotate at the same rate
- Direction is determined by the speed of the two wheel groups
- To move straight all motors rotate at the same rate
- Forward and Reverse are determined by positive and negative values
- Turning is achieved by varying the throttle on the right and left
Example
The Skidder class has the following API, as you can see all of
the movements we desire can be accomplished with a single API call
skidder.set_throttle(left_throttle, right_throttle),
does not get much simpler than this!
skidder = Skidder()
# Move forward at full speed
skidder.set_throttle(1, 1)
# Move in reverse at half throttle
skidder.set_throttle(-0.5, -0.5)
# Turn right arch at 60%
skidder.set_throttle(.6, 0)
# Turn left on a dime (and fast) at 70%
skidder.set_throttle(-0.7, 0.7)
# Stop
skidder.set_throttle(0, 0)
# Coast
skidder.set_throttle(None, None)
With this simple library we can control the most simple of Mobile Robots (or maybe a forklift).
Note About Speed and Accuracy
This library is very simple, it only knows how to control the percentage of potential load the motors will be provided. The Adafruit library appropriate uses the term throttle, because we can not know the speed of the motor without some form of external feedback.
Can NOT Directly Control Speed!
Of course, the library does not know what motors are being used, how they are geared (how the gear ratio’s are set). Even if we knew the motors, we still do not know, directly the load being supplied which will directly effect the rotation of the motors.
Nor Control Turn Radius
It follows to reason that the inability to control speed directly also means that we can NOT control the degree of our turns! For example a faster spinning motor will rotate a chasis more degrees than a slower rotating motor. This can be changed simply by the power supply being used, including the charge if batteries are being used.
External Feedback Required
This is not such a big deal when the vehicle is controlled by a Human, the humans makes control decisions in their brains and translate that to DATA INPUTS, from a joystick or keyboard possibly.
Computer Controls Require External Feedback
For the reasons stated above, Skid steering requires external feedback to be accurately used with computer automated controlers, like otto’s magoo.
The possibilities for external controls are many! Including distance sensors, computer vision, radar, maps of many varieties, etc. The choices are so vast that we would be remis to become too committed to one or two technologies.
For that reason our Raspberry Pi Device Controller (rpid) project was created to allow for external feedback of incoming new state combined with existing state measured against the desired state creates a new objective that will in turn be translated into a Control OUTPUT to the skid channel with a left and right throttles to effectively control the vehicle.
Skid Steering Demonstrated
Here are some diagrams and a video demonstrating skid steering in real life:
Todo create skid steer video
Todo create skid steer diagrams
Disclaimer ~ Semantics of the term Wheel
[1] Disclaimer ~ I will often use the term wheels in a very generic sense, sometimes the use of wheels is specificly to wheels, sometimes I will use it generically to include [tank tracks] or other types of elements that rotate to move the vehicle.
As distinguished from legs or other types of mobility enablers.